House outlook for 2013 (Take 1)

August 30th, 2012, 12:00pm by Sam Wang

A long-term outlook based on conditions through mid-August (pre-Ryan/Akin/convention) shows a possible loss of control of the House by Republicans (Democratic takeover, 69% probability). The 2013 seat margin is headed to being narrower in either direction than the current Congress. An election based on today’s conditions would give Democrats 50-51 seats in the Senate and ~224 seats in the House — a knife’s edge in both chambers. This picture may change.

Caveat #1: The quality of House district data is unlikely to be as good as it was for the last two cycles because polling is down. But perhaps our data source (HuffPost/Pollster.com) or another aggregator will know better. This is a situation where a more complex approach for filling in missing data might help.

My first estimate will therefore be based on the national popular Congressional preference. A simple inspection of election returns for 2000-2010 shows the well-known fact that the D-minus-R House popular vote share is strongly related to the corresponding margin of House seats: This graph shows that each 1.0% of popular-vote margin translates to a 6.0-seat advantage. Also, the intercept is nearly at the origin, i.e. no structural advantage for either side. A nearly-tied popular vote would translate to a nearly-tied House. So: 6.0 seats per percentage point.

However, note that individual data points deviate from the fit line by 7 to 17 seats in either direction. That is the first source of uncertainty, equivalent to 1-3 percentage points in national vote.

Next, let us use the generic Congressional ballot to estimate national popular vote. I will apply the Wisdom-of-Pollster-Crowds principle that has served so well in Presidential and Senate races, and assume that aggregating polls will approximate the actual popular preference.

Request to readers: Some have claimed that the generic-preference ballot does not predict seat outcomes. If you have recent quantitative data on this subject that is appropriate (i.e. that spans multiple pollsters, and surveys the last few Congressional elections), please add it in the comments section.

Here are this year’s polls. The average is a Democratic margin of 2.0%.

Update: a commenter took issue with the inclusion/exclusion of pollsters. My general approach, as longtime readers know, is to accept all polls. The above procedure was done strictly for display purposes, given Pollster.com’s software capabilities. Sadly, they do not use medians, which addresses the outlier problem. The median of all available generic Congressional preference polls since 8/15 is Democrats +2.0 +/- 2.0 % (median +/-SEM, n=7) (SEM corrected as per correspondence with Alan Cobo-Lewis, link to come).

Considering the possibility of movement in opinion between now and November and the fact that seat outcomes don’t precisely reflect popular vote, the total 1-sigma uncertainty is about +/-4%. This leads to a prediction of R+2% to D+6% (68% CI). This corresponds to anywhere from a 12-vote Republican majority to a 36-seat Democratic majority. The probability of retained Republican control is 31%, a knife edge situation.

All of the analysis above comes from data taken before the addition of Paul Ryan to the national ticket. Since then, the two most recent polls show the Democratic lead expanding to 4-8%. If that held, of course it would change the outlook. I’ll have more on this as it develops.

There seems to me to be an enormous unwillingness to listen to Internet/Robo Pollsters in the USA. You Gov, Ipsos etc. poll in other countries and have good records. Ditto Angus Reid who had the 2nd best prediction in the Wisconsin recall. How many more elections will it take before yourselves, TPM, Pollster etc. will consider using online? There are not enough media oulets left to fund pollsters and those non American media outfits still standing like Reuters and the Economist will fund on line polls since they are cheaper and appear to be more accurate.

Chuck – Read the post again. It was for display purposes, and also because Pollster does not calculate medians, which I always use. (Medians are so powerful for this application! Everyone should use them.) Your objection does not change anything.

RCP, 538, and Pollster are aggregators, not pollsters. So am I, and I have been doing it since 2004. However, thank you for your input.

Do you factor in something like the Democracy Corps poll of swing districts (I don’t think it conflicts — in fact, they’re coming to a similar conclusion, or were as of a month ago) in the generic preference #s or is it apples/oranges?

Chris R – thank you for writing. Everything is in the post. I try to avoid “factoring in” anything, unless it is statistically simple to handle and has multiple sources to allow aggregation. The methods are as close to transparent as possible, i.e. you could replicate most of the work in 10 minutes yourself.

In this case the generic Congressional preference poll is a simple quantity measured by multiple organizations. Swing district data would be great, and may be available from commercial aggregators in the coming weeks. We rely on Mark Blumenthal and Co. at Pollster for our data feed, and I read RCP often.

It is strange that the “seats vs votes” graph passes through the origin given the R advantage in Cook PVI. In short, city districts are nearly “pure D” zones, while the ‘burbs are just “mostly R” – thus at 50/50 vote the R’s should control the house 239 to 196.http://en.wikipedia.org/wiki/Cook_Partisan_Voting_Index#By_congressional_district
I can’t imagine the decennial gerrymander did anything but increase this effect.
But the data is the data, and I see nothing wrong with your approach.

Sam – Don’t even question linearization aloud or you may destroy all of what little math exists in the social sciences … And never ever ask a social scientist “what are the assumptions underlying the validity of the standard formula for a linear regression”.

It’s all in the post. Median and uncertainty fed into t-distribution for probability, same as EV analysis for single states. Uncertainty as described in post. 6.0 seats per percentage point margin. Recent polls or all 2012 polls give same answer.

That’s it.

The hardest parameter is how to account for asymmetry in the distribution of the amount/direction of movement between now and November. I am concerned about this parameter. For this reason the seat margin is a rather hard thing to estimate.

Each of the intrade contracts is just ten bucks notional, so one would hope that some grad students could both put a little money in their pockets while at the same time improving the market observable probabilities for the rest of us.

Maybe someone has mentioned it, but not that I saw. Aren’t years ending in 2 likely to differ structurally from other years? To wit, don’t we have to worry about redistricting here? Don’t know if the data exist before 2002, but it seems it would be useful to find a way to adjust for party control of state redistricting processes. Perhaps you could do that at the national level by just counting the number of house seats subject to GOP control over redistricting. Then you chew up only one df…..